Crossed field high frequency electron discharge apparatus



Sept; 26, 1956 R, 1.. BELL ETA! CROSSED FIELD HIGH FREQUENCY ELECTRON DISCHARGE APIARATUS v 4 sneets sheet 1 I Filed July :5. 1981 INVENTORS N U N H EM N BKM R Q Li DNH .T LOP A A Jf WW RGJ R. L. BELL ETAL Sept. 25

CROSSED FIELD HIGH FREQUENCY ELECTRON DISCHARGE APPARATUS 4 sheets-sheet 2 Filed July 5. 1961 INVENTORS RONALD LBELL GORDON 5. KING g JOSEPH K. MANN 1%;

ATTORNEY:

Sept.20,19fi5 R. 1... BELL ETAL 3,274,451

GROSSED FIELD HIGH FREQUENCY ELECTRON DISCHARGE APPARATUS Filed July 5, 1961 4 Sheets-Sheet 5 "Ji -f S 2 9 mvEmoRs 7 F Y4 RONALD L.BELL- GORDON 8. KING JOSEPH K. M

74 AT'ILORNEY Sept? 1955 R; L. BELL ETAL 3,274,431

Y CROSSED FIELD HIGH FREQUENCY ELECTRON DISCHARGE APPAREKYUS Filed July 5. 1961 4 Sheets-Sheet mvEw'roRs RONALD LBELL. GORDON s. KINO JOSEPH K. MANN B V TTORNEY t? n c r it to list. out in and Goruo DZSCEL Rail, w

Ronald L.

K. ML.

rinte i 'kilomcgncycle per second range is scaled down so that the nppnratus will operate at still higher frequencies, the power output of the tube falls off, the parts become overheated, and some of the ports are so small that they cannot be readily fabricated. These conditions are also true when the size of a cross-field RF. appuratus is scaled down to operate 'nt higher frequencies since, for example, the spacing between the negative sole electrode and the positive slow-wnve electrode must be made proportionately smaller than in lower frequency tubes, making the problem of voltage breakdown acute. Also, high beam currents cannot be achieved because the ellective cathode area will diminish rapidly as the frequency increases since t -e effective cathode length, traditionally a function of nti elength, decreases. At the same time, the'ci'liciency of the tube falls oil due to the decreased beam current driving a decreasing circuit impedance. The increased space charge density in the electron beam necessary to produce high powers in a smaller tube combines with the above eliccts to depreciate the over-all tube performance quite severely.

There are also fabrication ditliculties inherent in reproducing the fine mechanical detail required in tubes that must operate well over 10,000 megacycles per second. Thus, new methods for generating electron beams must be found which ease the problem of cuthods loading in existing scaled down guns of the conventional design wherein the length is a function of wavelength. Also, a new slowwnve circuit must be utilized which provides a wide interaction space so that higher beam currents can interact with the waves traveling on the slow-wave circuit without requiring an increase in space charge density of the beam.

Most of the problems which are encotintered when one tries to increase the operating frequency of an electron tube are also encountered when one tries to increase the power output of tubes without increasing the operating frequency, such as the problem of more efficient cooling and the problem oi voltage bi'cililCiOWfl.

The principal object of this invention is to produce a cross-field tube with higher elficiencies and higher output.

power, especially when the operating frequency is in creased.

One feature of this invention is an electron gun having an emitting surface disposed parallel to the electron beam with the length of the emitting surface in the direc-- tion ofthe beam greater than the length between the pointsoi origin of a cycloidnl period followed by any electron. I

Another feature of this invention is an electron gun wherein the electrons are accelerated into a nondlniform electric field so that electrons emitted ht one end of the gun are not recaptured at the other end Qfliili. gun.

Another feature of this invention is a gradient cnth-' ode wherein the potential varies uniformly iirom end to end of the cathode.

Another feature of this invention is an electron gun wherein the electrons are accelerated into an increasing electric field.

Another fenture of this invention is a slow-wave stru cture having tapered fingers to increase strength and cooling rate.

Another feature of this invention is an improved structure for maintaining dimensional tolerances at all opcrnting temperatures.

Another feature of this invention is an improved sole to retain the electron beam dense and close to the slowwzrve structure. 1

Another feature of this invention is an improved collector for collecting .a high density beam within a. very small space.

These and other objects and advantages ,will become apparent upon ape-rusnl of the following specification taken in connection with the accompanying drawings wherein:

FIG. 1 is a longitudinal cross-sectional view of a typicul cross-field tube employing the features of this invention,

FIG. 2. is a cross-sectional view taken on line 2 2 of FIG. 1 in the direction ot the arrows,

FIG. 3 is a cross-sectional view taken on line 3--3 of FIG. 1 in the direction of the arrows, l

FIG. 4- is a crossscctionnl view taken on line 4-4 of FlG l in the direction of the arrows,

FIG. 5 is a viewtzilten on line 5-5 of FIG. 1 in the direction of the arrows,

P16. 6 is an enlarged detail view of the region housing the electron gun shown in FIG. 1,

FIG. 7 is a cross-sectional view taken on line 77 of FIG. 6 in the direction of the arrows,

FIG. 8 is r view of the slow-waive stricture taken on line 8-8 of FIG. 1 in the direction of the arrows,

FIG. 9 is an alternate embodiment for the electron gun shown in FIG. 6,

FIG. 10 is a view taken on line 1t)-1ti of FIG. 9 in the direction of the arrows,

FIG. 11 illus rates the equipotcntinl lines for the cathode surface in FIG. 9 and trajectories of electrons leaving Y the cathode, and

FIG. 12 is an enlarged view of the interdigital line forming the slow-wave structure of the tube shown in FIG. 1.

Although the tube illustrated is a backward-wave oscillntor employing a crossed-field, the teachings of this invention may be used in an amplifier.

Referring to the drawings and to FIGS. 1 and 2 in particular, the crossed-field tube embodying the present invention has a rectangular envelope 12 with magnetic pole pieces 13 and 14 disposed in opposite side walls 16 and 17, respectively, of the envelope. The envelope has a pinched-oil exhaust tubulation 18 and two waveguides 19 and 21 protruding therefrom. Cernmic windows 22 and 23 rtrc each sealed at their periphery by suitable vncuum tight seals 24 to the interior walls of the waveguides 19 end 21, respectively, and within the envelope the wavebetween the two pole pieces 13 nnd14 and within a magnetic field that may be formed by a permanent horseshoe mngnet or an electromagnet (not shown) fixed to the pole pieces 13 and 14. Extending from the envelope 12 adjacent the electron gun assembly 27 is an assembly 31 comprising an alternate stack of metal andcerrtmic 'rings, metal ring32 serving as a terminal for nnnnode 33 of the gun assembly 27 and metal rings 34 anrl'36 serving as terminals fora cathode heater 37' (more clearly p the interior of the tube.

3,2 3. which is positioned within a cathode holder 38 which serves to support a cathode .shown in FIGS. 6 and 7) emitter 39. A metal ring 41 (FIG. 1) serves asthe terminal for the cathode 39, while the terminal for the sole 29 is a cup-shape member 42 closing the end of the ass'crnbly3l. t V

The electrodes which make up the gun. assembly 27,

' the sole 29, and the collector" assembly 28 are mounted on a ceramic bar 43 (shown in detail in FIG. 4) which is mounted on the side wall 16 parallel to the face of the pole piece 13. The ceramic bar 43 has at least three sole 29 and the slow-wave structure26 or the electrons accurately spaced lugs 49 and 49' (FIG. 5) which mate with the groove 44 and lug 49 which mates with groove '46 on the bar 43 accurately. locating the anode with respect-to the bar. A bolt 50, suitably insulated, screws into the side wall 16 to hold the anode in place. The cathode holder 38 has lugs 51 and 51 which mate with the grooves 47 and 44, respectively, in the ceramic bar 43 to accurately locate the cathode. A ceramic cathode enclosure 52 mates with pins 33' (FIG. 6) on the cathode holder 38 and holds the cathode holder 38 against the ceramic bar 43 electrically insulated from the other parts in the tube. The cathode enclosure 52 is held in place by a spring strap 53 (FIG. 5) extending from the bolt 50.

A surface. 54 (FIGS; 6 and'7) on the anode is preferably inclined at a slight angle to the direction of the beam and faces an emitting surface 55 on the cathode 3-9 to form a wedge-shape chamber therebetween.

By means of the a terminals 32 and 41 and. a power source, not shown, the

anode is maintained ata positive potential with respect to the potential on the'cathode. The strengthof the electric field created by this potential difference is greater between the surfaces 54 and 55 near the sole 29 where the surfaces are closest to each other, The direction of the magnetic field created between pole pieces 13 and 14- is oriented such that the electrons emitted from the cathode 39 fol- Thus, the surfaces 54 and 55 can be so arranged to provide an electrical field which causes all the emitted electrons to have cycloidal periods of different length which all crest at the same plane normal to the beam or have at this plane. The electrons are then extracted from the gun region to form an electron beam. An electron beam having a much higher density than in the prior art is produced since the length. of the cathode emitting surface '55 aligned with the beam neednot be limited to less than the length of thesmallest' cycloidal. period to prevent cathode recapture of electrons by the cathode. The electron gun of this invention causes the electrons emitted their velocity vector substantially parallel to the beam furthest from the slow-wave structure to'form longer cycloid periods so that they will not be recaptured by the cathode. Since the beam is at a relatively high density, the cathode holder 33 has upwardly protruding non-emitting ridges 56 (side hats) which extend towards the anode surface '54 to prevent the electrons from travcling sidewarclsin the direction of the magnetic field.

This arrangement of anode-and cathode surface to form a tapered chamber is also useful in low density beams and short length cathode since almost all the electrons in the beam are traveling ina direction parallel to o'ne another after they leave the gun.

' 1 The electrons of the beam can be accelerated to higher can enter this space having the same energy as in the gun assembly 27. In this embodiment, the electrons are further accelerated by tapering the space between the sole 29 and the slow-Wave circuit 26 so that the electrons move into an increasing electric field formed by sloping surfaces 57 (FIG. 6) on the negatively biased sole and sloping surface 57' on a grounded electrode 58 that is provided between the end of the anode 33 and the slowwave circuit 26 to shape the electrostatic field 'in the region betwcen the gun assembly 27 and the slow-wave circuit 26.

The surface of the sole 29 (FIG. 4) facing the slowwave structure is formed with protruding fins 59 over the entire length thereof and disposed parallel to the beam to reduce secondary electron emission. Protruding side hats 60 on the side edges of the sole 29 are disposed directly opposite an interdigital'line 61' formed in the slow-wave circuit 26, the side hats preventing the earn from spreading so that optimum RF. field-beam interaction is obtained.

The support structure between the sole 29 and the slowwave circuit 26 iinimizes the effects of the different thermal expansion coeflicients of the tube parts in the following manner: the sole 29has a laterally protruding ridge 62 which has a surface 63 that faces in the same direction as the face of the sole formed .by fins 59 and is accurately positioned parallel to the face of the sole. The ridge 62 engages the horizontal groove 44in the ceramic bar 4-3 for accurately positioning the sole 29 with respect to the bar 43. Bolts 64, suitably insulated, screw into the side Wall 16 and retain the sole in place. .As i pointed out above, the top edge-surface 47 of the bar 43,, i

being accurately spaced from grooves 44 and parallel thereto, engages the slow-wave circuitZrS at a ledge 67 that is accurately spacedfrom the interdi'gital line 61. The surface 45 of the ceramic bar 43 is urged against the ledge 67 and the ridge 62 on the sole is urgedagainst the side of the groove 4-4 by a plurality of spring clips 63 acting on ceramic wedges positioned under the sole 29. Suitable bolts 71 retain the spring clips 63 in place. This arrangement of parts allows the dissimilar materials of the components to thermally expand without appreciably affecting the spacing etween the sole 29 and the interdigital line 61 since the reference portions 45 and 67 are disposed as close as possible to the reference portions 44 and 63. A lug 62 (FIG. 5) on sole 29 engages a vertical slot 47 disposed parallel to slot. 47in bar 43 i to align the sole with respect to the cathode 39.

Since a typical tube of the present construction prov duces very high frequency oscillations at high power, such as on the order of 15,000 megacycles, at watts of continuous power output, the high density beam may cause the yery small interdigital line 61 to overheat. Referring to FIG. 12, fingers 72 of the interdigital line 61 are cooled and made stronger by providing an inner tapered portion 73 which is adjacent a support Willi. 7d

. of the slow-wave circuit 2.6 and which forms preferably one-halt of the finger 72. The fingers 72 havejari end i portion 76 with a substantially uniform cross section,

and a lower edge 77 of the end portion 76 is recessed inwardly of the lower surface of the interdigital line 61.and

away from the electron beam to minimize electron interception, This arrangement of an inner taper portion 73 extending only to the midpoint of the finger 72 and an 7' I end portion 76 with a substantially uniform cross section provides minimum overlap between afinger72 and an adjacent finger 72 to minimize capacitance between the two proves cooling and strength.

The tube illustrated is a backward wave oscillator which and produce high interaction impedance. The large cross section of the inner portion 73 near the side Wall 74 imconverts the beams kinetic energy to high frequency energy according to well-known physical principles. The high frequency wave is coupled out of the tube. through ,line 61 has two 90 bends, the inner surface electrode 58 is tapered with a notch therein'as shown to the waveguide 19 (FIG. 1). The .waveguide 19 has a i rectangular cross section with the side walls 74 which support the fingers 72 forming opposing walls of the waveguide. The first finger 72" (FIG. 8) in the interdigital line 61 extends into the waveguide 19 to form an elongated tapered fin 78 (FIGS. 1 and 2) which isdisposed.

perpendicularly on one of the walls7=t of the waveguide to form the transition between the waveguide 19 and the interdigital line 61. In a backward wave oscillator, wave reduce reflections. ,A corner part 79 in waveguide 21 is constructed similar to the electrode 53.

Since the interdigital line 61 heats up considerably during operation, cooling water is circulated by tubes 31 and 82 into a vessel 83 which makes contact with side walls 74 (PI tlof the interdigitalline 61.

In the collector assembly at the eud'of the tube (FIGS. 1 and 3), the sole face fins 59 which fins substantially 'reduce secondary emission are continued on a sole extension plate 84 which is disposed in a recess 35 formed in the block 36 of collector assembly 26. Block 86 is bolted in place by suitably insulated bolts 86'. The

sole extension plate 84 doesnot have side hats similar out of the vacuum envelope. The insulator 93 and the ceramic bar 43 insulate the collector 28 from the body of the tube so that a depressed potentialcollector may be provided in the tube.

The electrons when they are emitted from the surface of the cathode 39 are attracted by the anode 3.3 towards surface 54. The pole pieces 13 and 14 orient the magnetic field to cause the electrons to travel a cycloidal path and travel towards the space betweenthe slow-wave. circuit 26 and the sole 29. The cycloidal path of each electron varies since the electric field between the cathode and the anode varies, and the electric field can be so shaped to cause all the electrons to form cycloidal paths that will be closest to'the anode at the edge of surface 54 which edge is near the slow-wave circuit 2 6. At this point the electric field strength is adjusted by sloping surfaces 57 and 57' to cause the electron to travel a path normal to the electric and magnetic field.

.The side hats 56 onthe cathode holder 38 and the side hats 69 on the sole prevent the beam from spreading laterally and retain the electrons close to the fingers 72 where the RF. field isstrongest.

After the electrons interact with the RF. field they are collected by the collector. Since the beam current is relatively high and confined by the side hats 69 on the poles, the electrons in the beam must spread so that they can be collected over a large area to prevent overheating damage. Since the beam is traveling at a high velocity, considerable length is required before appreciable beam spreading is accomplished. Tov aid beam spreading the side hats are removed from the sole after the interdigital line 61 terminates. The electric field between the plate 84 and collector housing is of a valuewhich causes the beam electrons to turn around the end of the plate electron gun assembly 27 is shown which also produces 'a higher density beam than a gun of the prior art. The

tungsten wound around threesapphire rods 193.

electron gun assembly 27 has an emitting surface 101 that has a gradient potential, that is, there is a potential difference between one end of the cathode with respect to the other end. A gradient cathode in this embodiment is formed, for example, by wire 102 such as thoriatcd- The wire 182 is heated by a battery 104 that is polarized to make the end of the emitting surface 101 near the sole 29 negative with respect to the other end of the emitting surface 161. The sole 29 is biased negative with respect to the cathode by a battery 106, while the anode 107 which can be part of the slow-wave structure 26' is biased positive by battery 108. The equipotentia-l electric field lines between the sole 29', the cathode surface 191 and the anode 107 are shown schematically in FIG, 11 by dotted lines 111 which will terminate on the cathode surface 101 asshown. The electrons emitted from the positive end of the surface 101 will follow a cycloidal path 112 and will not be recaptured bythe cathode. Other electrons will follow other paths-such as path 113. Thus, all the electrons emitted by the cathode will eventually form the electron beam and interact with the slow-wave structure. i T

A gradient cathode has an additional advantage in that by reversing the potential to the ends of the wire 192, the emissive current is interrupted or sharply decreased because almost all the electrons will return to the cathode. Then, if the potentialv ofthe'DC. power supply 104 is alternately reversed, the beam current will be modulated. In this type of application, the voltage of the DC. power supply must be preferably lower than. when the beam 84; and double back. towards the electron gun so that they have time to spread before they are collected without increasing the iength ofthe tube unnecessarily; Referring to FIGSL9 and IO'thefre is shown another embodiment ofthe present invention. A crossed-field is unmodulated to prevent overheating of the cathode. Since the voltage of power supply 104 islow, somewhere between 25 v. and v., the switching of potential can be done rapidly thereby reducing noise.

Since many changes could be made in the above construction and many apparently widely different embodiments of this invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is: l

1. A high-frequencyapparatus comprising an electron gun said gun being adaptedand arranged to generate an electron beam, at slow-wave structure, and a collector, said slow-wave structure comprising an interdigital line includ ing a plurality of fingers mounted on spaced parallel sup port walls, said fingers being interdigitated, adjacent fingers being afiixed to opposite walls at theone end of said fingers and free at the other end thereof and said fingers being tapered along the length thereof, said taper being such that the cross-section of said fingers is con- ,siderably reduced at the mid-portion of the length thereof relative to cross section at the fixed end thereof, said fingers having a relatively minute change in cross section from approximately said mid-portion thereof to said free end thereof.

2. The high-frequency apparatus of claim 1 wherein fingers is reduced.

3. A crossed-field high-frequency apparatus comprising an electron gun, a slow-wave circuit, and a collector,

said electron gun having a cathode with a planar emitting surface facing an anode havinga planar'anode surface,

said planar anode andsaid planar cathode surfaces each having 'an edge located near. said slowwave circuit and o l a u ron gun and said collector, the spacing between said sole and said slow wave circuit. being tapered from a'larger ode surface at the edge of said cathode near said slovwave circuit is smaller than the distance between said the opposite edge of said cathode, the electric held strength established between said cathode planar emitting surface and said anode planar surface increasing from the upstream to the'downstream edges of said cathode planar emitting surface such that electrons emitted from said cathode planaremitting surface are accelerated into an electric field of increasing electric field strength between the upstream and downstream end portions of said gun.

4 A crossed-field high-frequency apparatus comprising 'anelectron gun, a slow-wave circuit, and a collector, said electrongun having a gradient cathode electron emission surface having a near end edge thereoi positioned near said slow wave structure and a far end edge thereof spaced further from said slow wave structure than said near end edge and wherein the end edge of the cathode near the slow-wave structure is'at a negative potential with respect to the other end edge of the cathode.

5. A crossed-field. high-frequency apparatus comprising an electron gun, a slow-wave circuit, and a collector, said electron gun having a cathode having a generally planar emission surface facing an anode havinga generally planar surface said anode and cathode forming a wedge shaped chamber such that a non-uniform electrostatic field is formed therebetween, said non-uniform elecplanar anode surface and said planar cathode surface at i tro static field formed between said anode and cathode increasing from the upstream end of said cathode to the downstream end of said cathode.

6. A crossed-field high-frequency apparatus compris 7. A crossed-field tube comprising an electron gun for forming and directing an electron beam along a beam path, a slow wave circuit disposed parallel to said beam path, a sole'electrode disposed parallel to said beam path andon the opposite side thereof from said slovv'wave circuit, a collector, and means for maintaining an accurate spacing between said slow Wave circuit and said sole electrode, and wherein said means comprises a ledge on'saicl slow-wave circuit accurately positioned from the interaction means on said slow-wave circuit; a protruding ridge on said sole electrode; a ceramic bar having a groove accurately disposed from an edge, along the length thereof said edge being disposed against said ledge and V said protruding ridge being disposed within said groove;

and sccond means for urging said sole towards said slowwave structure whereby said ridge bears against said groove and said edge in turn bears against said ledge.

8. A crossed-field tube comprising an electron gun for forming and directing an electrontbeam along a 'pre.-

determined electron beam path, a slow-wave circuit disposed parallel to said beam path on one side of said beam path, a sole electrode disposed parallel to said beam path and onthe opposite side thereof from saidslowwave circuit, and a collector, said slow-wave circuit including two sidewalls having 'an interdigital line formed along their edge that is disposed adjacent said sole, means on said sole for retaining the electrons in the ,beam close to the said interdigital line, said means com- ;3 prising side hats disposed on each side of said sole along the length of said sole, said side hats being disposed closer to said slow wave circuit than any other part of said I "sole, said sole and said slow wave circuit extending along I said predetermined electron beam path between said elecspacing therebetwcen at the electron gun end of sa sole and said slow .wave circuit to a relatively smaller spacing therebetween at a distance along said sole and said slow wave circuit removed from said electron gun end.

9. A crossed-field tube comprising an electron gun for forming and directing an electron beam along a predetermined electron beam path, a slow-Wave circuit disposed parallel to said beam path, a sole electrode having a pair of side hats disposed along a portion of the length thereof disposed parallel to said beam and on the opposite side thereof from said slow-wave circuitand a col ector, said collector being disposed at the end of said sole opposite said gun, said collector spaced from the sides, end and bottom of said sole, and means for causing the electron beam to turn aroundthe end of said solev and continue between SQltl sole and said collector until the beam spreads sufficiently to be collected, said sole'elect'rode having the side hats removed in the vicinity of said collector region.

It A crossed-field tube comprising an electron gun for forming and directing an electron beam along a pred termined electron beam path, a slow-wave circuit disposed parallel to said beam path, a sole electrode disposed par-- allcl to said beam andon the opposite side thereof from said slow-wave circuit, a collector, and first means. for maintaining an accurate spacing between said slow-wave circuit and said sole, said slow wave circuit comprising an in'terdigital line including a plurality of fingers mounted on two parallel support walls, said fingers being tapered and fixed by the larger cross section thereof to each of the support walls, said electrongun having a cathode with an emittin surface facing an anode surface and forming a non-uniform electrostatic field therebetwecn, said collector being disposed at the end of said sole opposite said gun, and spaced from the side, end and bottom of said sole, and second means for causic" the electron beam to turn around the end of said sole and continue between said sole and said collector until the beam spreads sumciently to be collected. I

11. The crossed-field tube of claim 18 wherein said first means'comprises a ledge on said slow-wave circuit accurately positioned from the interaction means on said slow-wave circuit; a protruding ridge on said sole electrode; a ceramic bar having a groove accurately disposer from an edge thereof, said edge being disposed against sa d ledge and said protruding ridge being disposed within said groove; and third meanstfor urging said sole towards said slow-wave circuit whereby said ridge bears against said groove and said edge in turn bears against said led e.

12. A crossed-field tube comprising an electron Sun for forming an electron beam, at slow-wave circuit disposed parallel to said beam, a sole electrode disposed parallel to said beam on the opposite side thereof from said slow-wave structure, said sole electrode having side hats disposed thereon along a portion of the length thereof, a collector, and first means for maintaining an aceurate spacing between said slow-wave circuit andsnid sole, said collector being disposed at the end of said sole opposite said gun, said collector spaced from the sides,

end and bottom of said sole, and second meansfor'causmg the electron beam to i and continue between said sole and said collector until the beam spreads sufiieiently to .be collected,,said sole electrode having the side hats removed inthe vicinity of said collector.

13, A crosssed-field tube comprising an electron gun for forming an electron beam, :1 slow-wave circuit dis? posed parallel to said beam, a sole e'lectrodedisposed parallel to said beam on the opposite side thereof from said, slow-wave ClfCult, a collector; and firstmeans forrnaintaining "an accurate spacing between-said circuit and said sole, said slow-wave circuit comprising an interdigital line including a plurality of fingers mounted on parallel supportwalls; said fingers eing tapered and fixedby the turn around the end of the sole 9 larger cross section thereof to the support wall, said electron gun having a cathode with an emitting surface whose length in the direction of beam travel is greater than the length of a cycloidal period followed by any electron emitted by the emitting surface and the crest of the cycloidal period followed by the same electron, said collector being disposed at the end of said sole-opposite said gun,

said collector being disposed opposite a portion of each of three sides of said sole and at its end, second means for causing the electron beam to turn around the end of the soleand continue between said sole and collector until the beam spreads ,sufiiciently to be collected, and

said first plurality of fingers defining a first plurality of-v coupling means for coupling high frequency energy with said-slow-wave structure.

14. The crossed-field tube of claim 13 wherein said first means comprises a ledge on said slow-wave structure accurately positioned from the interaction means on said slow wave structure, a protruding ridge on said sole elec trodc, a ceramic bar having a groove accurately disposed from an edge, said edge being disposed against said ledge, said protruding ridge-being disposed within said groove, and third means for urging said sole towards said-slowwave structure whereby said ridge bears against said groove and said edge in turn bears against said ledge,

said electron gun having a cathode with 'a planar cmitr ting surface facinga planar anode surface, and the distance between the surfaces at the ed e ear theslow-wave structure is smaller than the distance between the surfaces at the opposite edge thereof.

15. The crossed-field tube of claim 13 wherein said first means comprises a ledge on said slow-wave structure accurately positioned from the interaction means on said slow-wave structure, a protruding ridge on said sole electrode, a ceramic bar having a groove accurately disposed from an edge, said edge being disposed against said ledge, said protruding ridge being disposed within said groove, and third means for urging said sole towards said slow-wave structure whereby said ridge bears against said groove and said'edge in turn bears against said ledge,

said electron gun having a gradient cathode wherein the end of the cathode near the slow-wave structure is at a negative potential with respect to the other end of the cathode.v

16. A high frequency apparatus comprising, an electron gun, for generating and directing an electron beam along a predetermined electron beam path, a slow wave circuit, and a collector structure, said slow wave circuit comprising an interdigital structure including a'pairof spaced substantially parallel support walls wherein one of said parallel support walls has a first plurality of fingers atfixed thereto, each of said first plurality of fingers being directed along the length dimension thereof towards and spaced from the other of said parallelsupport walls, said first plurality of fingers being spaced from said electron beam path, said fingers defining a side thereof furthest removed from said electron beam path and said fingers I defining a side thereof closest to said electron beam path,

spaced parallel planes, said first plurality of spaced parallel planes being substantially perpendicular to each of i said spaced parallel support walls, and wherein the other of said pnrallel support walls has a second plurality of fingers aliixed thereto, each of said second plurality of fingers being directed along the length dimension thereof towards and spaced from the one of said parallel support walls, said second plurality of fingers being spaced from said electron beam path, said fingers defining a side thereof furthest removed from said electron beam path vand said fingers defining a side thereof closest to said electron beam path,-said second plurality of fingers defining a second plurality of spaced parallel planes, said second plurality of spaced parallel planes being substantially perpendicular to each of saidspaced parallel support walls,

.said first plurality of spaced parallel planes being interdigitated with respect to-said second plurality of spaced parallel planes, said fingers having an, enlarged crosssection at the portion arfixed to said supported wall and said fingers being tapered over a portion of the length thereof such that a substantially reduced cross section exists in the vicinity of the midportion of thefingcrs taken along the length thereof, said fingers being substantially non-tapered from said mid portion to said free end along the side thereof furthest from the electron beam path.

17. The device as defined in claim 16' wherein said fingers have the sideclosest to the electron beam path tapered away from said beam path thereby decreasing,

electron interception by said interdigitated slow wave structure.

18. A crossed-field high frequency electron discharge gun, said electron discharge gun comprising a cathode with an electron emission surface, said emission surface having upstream and downstream end portions aligned along the direction of electron flow from said emission surface and means for establishing a potential gradient along the emission surface of said cathode between said upstream and downstream end portions of said emission surface, said gun further including means for establishing Crossed-electric and magnetic fields insaid gun region.

References Cited by the Examiner JAMES W. LAWRENCE, Primary Examiner.

' GEORGE N. WESTBY, DAVID J. GALVIN,

. Examiners. V. LAFRANCHI, Assistant Examiner. 

1. A HIGH-FREQUENCY APPARATUS COMPRISING AN ELECTRON GUN SAID GUN BEING ADAPTED AND ARRANGED TO GENERATE AN ELECTRON BEAM, A SLOW-WAVE STRUCTURE, AND A COLLECTOR, SAID SLOW-WAVE STRUCTURE COMPRISING AN INTERDIGITAL LINE INCLUDING A PLURALITY OF FINGERS MOUNTED ON SPACED PARALLEL SUPPORT WALLS, SAID FINGERS BEING INTERDIGITATED, ADJACENT FINGERS BEING AFFIXED TO OPPOSITE WALLS AT THE ONE END OF SAID FINGERS AND FREE AT THE OTHER END THEREOF AND SAID FINGERS BEING TAPERED ALONG THE LENGTH THEREOF, SAID TAPER BEING SUCH THAT THE CROSS-SECTION OF SAID FINGERS IS CONSIDERABLY REDUCED AT THE MID-PORTION OF THE LENGTH THEREOF RELATIVE TO CROSS SECTION AT THE FIXED END THEREOF, SAID FINGERS HAVING A RELATIVELY MINUTE CHANGE IN CROSS SECTION FROM APPROXIMATELY SAID MID-PORTION THEREOF TO SAID FREE END THEREOF. 